1. Field of the Invention
The present invention relates to the production of sealed and thermally insulated tanks incorporated into a bearing structure, particularly the hull of a ship intended for transporting liquefied gases and, in particular, for transporting liquefied natural gases with a high methane content, by sea. More specifically, the present invention relates to a panel for manufacturing such a tank and to a method of manufacture employing this panel.
2. Description of the Related Art
French Patent Application No. 2 724 623 proposes a sealed and thermally insulated tank incorporated into a bearing structure, particularly of a ship. The wall of the tank exhibits, in succession from the inside of the tank towards the bearing structure, a primary sealing barrier in contact with the product contained in the tank, a primary thermally insulating barrier and a secondary sealing barrier and a secondary thermally insulating barrier. The primary sealing barrier consists of metal strakes with edges turned up towards the inside of the tank, the said strakes being made of thin sheet with a low coefficient of expansion and being welded edge to edge via their turned-up edges to the two sides of a welding support, which is held mechanically on the primary thermally insulating barrier and constitutes a slip joint. The primary thermally insulating barrier, the secondary sealing barrier and the secondary thermally insulating barrier essentially consist of a collection of prefabricated panels fixed to the bearing structure, each panel being formed, firstly, of a first rigid board bearing a layer of thermal insulation and with it constituting a secondary thermally insulating barrier element, secondly, of a flexible or rigid covering stuck to practically the entire surface of the layer of thermal insulation of the aforementioned secondary thermally insulating barrier element, the covering consisting of a composite material the two external layers of which are fibreglass fabrics and the intermediate layer of which is a deformable thin aluminium sheet about 0.1 mm thick, the sheet forming a secondary sealing barrier element, thirdly, of a second layer of thermal insulation which partially covers the aforementioned covering and which is stuck thereto and, fourthly, a second rigid board covering the second layer of thermal insulation and with it constituting a primary thermally insulating barrier element. Tanks comprising prefabricated panels of a similar structure, in which the primary sealing barrier consists of corrugated aluminium or stainless steel strakes are also known.
The regions where two adjacent panels meet are filled in, so as to ensure the continuity of the secondary sealing barrier. More specifically, to ensure the continuity of the imperviousness of the secondary sealing barrier, provision is made for the adjacent peripheral edges of two adjacent panels in the region of the joins between panels to be covered with a strip of flexible covering comprising at least one continuous thin metal sheet, the strip being stuck to the two adjacent peripheral edges and, by its metal sheet, ensuring continuity of sealing.
The quality of the seal of the secondary sealing barrier is dependent on several factors, particularly on the quality of the bond between the strip of flexible sheet and the peripheral edges of the panels. In order to guarantee that the bond is of good enough quality, it is known practice, when bonding the strip of flexible sheet, to carry out a spew test involving:                fixing the panels to the bearing structure of the tank,        placing some adhesive tape on the panels fitted, adjacent to the region that is to be bonded,        bonding the strip of flexible sheet using an adhesive in such quantity that spew forms beside the flexible strip and partially covers the adhesive tape,        once the adhesive has cured, removing the adhesive tape,        examining the spew once the adhesive tape has been removed and validating the bond on the strength of this examination.        
This manufacturing and test method has several disadvantages. Between the prefabrication of the panels and their attachment to the tank, the surface condition of the flexible or rigid covering which forms a secondary sealing barrier element may become degraded, for example during transport, storage, or while the panels are being attached, because of contamination becoming deposited on the flexible covering or as a result of the effects of external conditions, for example temperature or ultraviolet radiation. A damaged surface may give rise to a poor quality bond. Before bonding the strip of flexible sheet it is necessary to place the adhesive tape on all the panels installed, and this takes a great deal of time. In addition, the back of the adhesive tape is generally covered with a non-stick material which may migrate and greatly disrupt the bonding of the strip of flexible sheet.